AshLua eval read operations can read field-policy-protected fields via aggregates

AshLua exposes Ash read actions to Lua scripts run through an eval action. A read call accepts an operation (list, min, max, first, sum, avg) that builds an ad-hoc Ash.Query.Aggregate over a named field and returns its raw value.

Ash field policies redact forbidden fields on returned records (replacing them with %Ash.ForbiddenField{}), but that redaction does not apply to aggregate values. A script could therefore read a field the calling actor's field policies forbid by requesting it as an aggregate instead of as a field. This includes fields that are public? true but restricted per-actor by a field policy, such as sensitive PII. The prior hardening only enforced the exposed-field allow-list (field visibility), which is a separate axis from per-actor field-policy authorization.

The fix authorizes the aggregated field against the resource's field policies, so aggregating over a field the actor may not see is refused or scoped to the rows where it is visible.

This issue affects ash_lua: from 0.1.0 before 0.2.2.